Metal matrix composites provide a relatively new way of producing parts with exceptional stiffness, resistance to wear and resistance to heat. The superior stiffness allows thinner materials to be used and thus reduces the size and weight of the part. Light weight is important in the construction of components for the aerospace industry and the electronics industry.
At the present time, the processes employed for fabrication of MMCs include a vacuum/low pressure infiltration, a high pressure squeeze casting process and a pressureless casting process. These processes have the following characteristics:
Low Pressure/Vacuum Infiltration Process
The process utilizes a combination of vacuum and a low pressure (typically less than 1500 psi) gas for metal infiltration. The process has been employed mainly for fabrication of small numbers of R&D samples. It is designed to be a laboratory-scale process and is not suitable for low cost, high throughput production on a commercial scale.
Pressureless Infiltration Process
The process relies on capillary force for the infiltration. To develop the capillary force, a wetting between aggregate and matrix materials is obtained through a chemical reaction. The process requires the use of special combinations of matrix alloying elements (alumninum with 1 wt. % Mg or higher), heated ceramic aggregates (700.degree. C. or higher) and gas environment (10 to 100 vol. % nitrogen). This process thus limits selection of matrix material, for instance Al-Mg alloys and is burdened by high manufacturing costs due to extended reaction periods. This process is additionally difficult from a process control standpoint due to the sensitive dependence of the process on surface condition and spontaneous wetting.
High Pressure Squeeze Casting Process
For the infiltration, this process utilizes high pressure (10,000 to 30,000 psi) using a large hydraulic or mechanical press. The process has the ability to infiltrate a wide range of aggregate and matrix combinations but is limited as to complexity and dimensional tolerance of the products produced because of limitations in die design imposed by the need for high pressure. The product manufacturing costs are also high because of high capital costs for the large press.
U.S. Pat. No. 4,920,864 generally discloses a closed-die pressure casting process to achieve infiltration of a matrix alloy into the interstices of a fiber preform. U.S. Pat. No. 4,777,998 shows die casting of metal parts possibly containing fibers of ceramic materials, but no attempt is made to utilize vacuum die casting principles.
It was against this background that the present invention was made.